1. Field of the Invention
The present invention relates to a frequency stabilizing device and, more particularly, to a carrier frequency stabilizing device in a digital mobile radio communication system.
2. Description of the Related Art
A conventional frequency stabilizing device is described in, for example, Japanese Patent Laid-Open No. 256010/1998 (JP-A-63256010). This frequency stabilizing device detects a received field strength of a received signal and controls an oscillation control signal to an oscillator in response to frequency error information when the received field strength surpasses a predetermined threshold. This technique results in very effective frequency stabilizing operation.
Another frequency stabilization device is described in, for example, Japanese Patent Laid-Open No. 256021/1988 (JP-A-63256021). This frequency stabilizing device improved the frequency stabilizing operation under Rayleigh fading by constructing two receiving portions and by achieving the frequency stabilizing operation based on a higher received field strength.
However, these frequency stabilizing devices do not achieve an accurate frequency stabilizing operation because accurate frequency error information is not obtained from a demodulated signal when the received field strength of the received signal is high but a modulated carrier does not exist. this condition may occur when, for example, there is interference from an adjacent channel.
Moreover, since the latter device needs the two receiving portions, the circuit is necessarily large.